Our studies are designed to improve the use of platelet transfusions in thrombocytopenic patients, particularly those with Acute Leukemia. Patients at Children's Hospital are randomized between two groups, prophylactic and therapeutic. Patients in the prophylactic group receive transfusions whenever the platelet count falls below 20,000 per mm to to the 3rd power; those in the therapeutic grous receive transfusions only for specific clinical indications. The long- term clinical courses of thses two groups are being compared. Patients often become refractory to platelet transfusions. We are studying the incidence of this phenomenon and correlating it with HL-A typing and the results of assays for lymphocytotoxic antibodies, leukoagglutinins, and complement-fixing antiplatelet antibodies. Refractory patients will be managed with platelets from HL-A-compatible donors. We are also studying methods to improve the liquid storage of platelets. Specifically, bags constructed of polyethylene appear to be markedly superior to those constructed of polyvinylchloride for storage at 22 degrees Centigrade. Furthermore, we are re-evaluating the possibility of storing platelets at 4 degrees Centigrade.